Hopefully you mean the AiNet and NOT the M-Bus. Obviously AiNet is the current bus that is used by Alpine devices. Only ancient euipment uses the M-Bus and those communications were cracked long ago. I'm only pointing this out so you don't start to look in the wrong place...
But you would make a lot of people happy if you reverse engineered the AiNet Protocol. (Oh, you might make a few people unhappy at Alpine, but screw them...) There are so many things I would like to connect to my HU, but Alpine has made it very difficult by keeping their interface so secret.
While you're at it, please consider also cracking their Amplifier Bus. That is a bus that allows you to display things like amp gain, power, temperature directly on your HU. It would be more useful for displaying other things like indoor/outdoor temp, car voltage, etc. but again Alpine doesn't want anybody to actually use the feature so they keep it secret.
Well hell fig it out when he looks at the plug that it aint m-bus. Hopefully it wont be long though. Cuz XM SUXKS right now